The art of making integral low density polymeric closed cellular structures from particulate expandable polymeric material is well known. Expandable polymeric materials include polystyrene, alkyl-substituted styrenes, copolymers of styrene, alphamethyl styrene, and the like. These materials can be made by forming a stable aqueous suspension of polymer particles and impregnating the suspended particles with a volatile aliphatic hydrocarbon such as petroleum ether, pentane, hexane, heptane, and the like. The expandable materials have from 3 to 30 parts by weight of the aliphatic or cycloaliphatic hydrocarbon incorporated therein. These expandable particles are generally referred to as "beads" and may take any shape such as spherical, oblong, irregular, ect.
Various means have been used for heating these beads or pre-expanded beads, to cause them to expand or further expand into an integral cellular polymeric structure or part. For example, hot water, infrared rays, and steam have been used to heat the beads in a mold and cause them to expand and conform to the shape of that mold. More recently, however, it has been found that expandable polymers such as polystyrene and the like can be heated by processes utilizing high frequency electrical energy. Generally, the molding of expandable polystyrene beads to form fused integral foam pieces or parts results in a closed cellular structure; that is, there is substantially no interconnection between the cells of the structure. This means that the water vapor transmission into or through such parts is low and the buoyancy of such parts is high.
High frequency electrical energy has also been used to bond webs of porous aliphatic polyolefin material of a thickness less than 1.5 mm. to a substrate, U.S. Pat. No. 3,738,886, and large cellular plastic parts, Soviet Plastics, July 7, 1962, pp. 29-31. In these methods, however, bonding of closed cell foam polystyrene was not achieved without the use of an adhesive.
Illustrative of processes for molding or fusing expandable polystyrene to an integral structure by the use of high frequency electrical energy are U.S. Pat. Nos. 2,998,501, 3,010,157, 3,060,513 and 3,242,238.
Low density expanded polymeric structures and material have been found suitable for use in numerous applications, including packaging, insulation, and the like.
An important new application for molded low density polymeric materials and, in particular, expandable polystyrene is in the vaporizable pattern molding processes for casting. Illustrative of these processes are U.S. Pat. Nos. 2,830,343, 3,314,116 and 3,157,942. In general, the process includes embedding in sand a vaporizable casting pattern in a flask. The pattern is typically made of expandable polystyrene or other vaporizable expandable polymeric material and having a porous refractory coating. It is embedded in a clean, dry unbonded sand which emcompasses and fills all of the cavities and recesses of the pattern. The pattern usually includes an integral runner for supplying or directing the molten casting material to the embedded pattern. The molten casting material is then poured through the runner and to the pattern. The heat of molten material vaporizes the polymeric material and the gases escape through the porous refractory coating and into the sand. A casting is thus formed taking the exact shape of the pattern. By this method, extremely complex castings can be made in a manner which provides not only a superior casting, but one which is prepared at a substantially reduced cost over conventional casting methods. In order to take advantage of these benefits, however, it is necessary to utilize a high quality expanded polymeric pattern, particularly with respect to surface quality and dimensions.
The production of low density polymeric structures or patterns generally must be carried out in a mold using conventional molding techniques. Moreover, many of the castings for which patterns are made are extremely complex such as engine blocks, manifolds, brake assemblies, and the like. Because of this complexity, expandable polymeric materials do not lend themselves to conventional molding techniques without considerable time and expense. To overcome the difficulties attendant with the complex patterns as well as conventional molding techniques and still maintain high quality, it has been proposed to divide the complex pattern into a number of component elements which are relatively simple to mold. These component elements are then bonded together to form the complex pattern.
While a high strength bond is not required, it must have strength sufficient to maintain the integrity of the pattern during normal production handling, such as placing it in the flask and withstanding vibrational forces used to settle and complete filling by the sand. Moreover, the bonding material must volatize at substantially the same temperature and at the same time. Otherwise, irregularities in or on the casting are created. Use of adhesives have the disadvantage that an adhesive line would be formed and serve as a partial block when pouring the molten casting material because of its relatively high density and resistance of vaporization causing irregularities in the casting. Also, many of the adhesives leave undesirable residues in the casting. Further, adhesives require time to set or cure. Hot wax has been utilized for bonding, but it has not been found generally satisfactory. Not only does it lack strength as a bond, but it is difficult to handle because of its very fast setting time. Accordingly, it is extremely difficult and/or expensive to prepare a complex pattern from component parts utilizing conventional bonding techniques.
The present invention provides a method for bonding foamed polystyrene parts without the use of an adhesive. The invention further provides a method which renders the surface at the joint of a quality substantially equal to the surface of the expanded part. The method of the present invention is adaptable for low cost, high production processes; thus, overcoming the disadvantages of conventional bonding techniques.